NFL owners have filed an unfair labour practice complaint against the American football players' union.
With no sign of progress in talks, the NFL's lawyers says the union has failed to bargain in good faith.
Despite a successful season with huge television ratings owners insist profit margins are being squeezed and the current agreement is no longer sustainable.
There are also disputes over drug testing, a rookie wage scale and plans to increase the regular season from 16 to 18 games.
The current agreement runs out on March 4 and if a new deal's not be reached by then a lockout's possible.
The NFL Commissioner Roger Goodell fears a fan backlash if that happens.
